Submersible pumping systems are often used in hydrocarbon producing wells for pumping fluids from within the well bore to the surface. These fluids are generally liquids and include produced liquid hydrocarbon as well as water. One type of system used in this application employs a electrical submersible pump (ESP). ESP's are typically disposed at the end of a length of production tubing and have an electrically powered motor. Often, electrical power may be supplied to the pump motor via an electrical power cable from the surface that is strapped alongside the tubing.
A motor lead is secured to the lower end of the power cable, the motor lead terminating in a connector that plugs into a receptacle of the motor. This connector is typically known as a pothead connector.
ESP motors have stators with slots. Insulated magnet wire is wound through the slots in a selected pattern. A sheet of an insulation material may be wrapped around each bundle of magnet wires within each of the slots. The magnet wires extend below a lower end of the stator in loops spaced around a longitudinal axis of the motor. An end bell insulation sheet is formed as a cylinder and extends around all of the loops. The loops are arranged in phases spaced longitudinally from the longitudinal axis. Phase-to-phase insulation sheets are rolled into cylindrical shapes and between the different phase groups. The magnet wires may be bonded in the slots with an epoxy. In one technique, magnet wire leads are spliced to upper ends of three of the magnet wires. The magnet wire leads extend from the upper end of the stator to internal contacts in the motor electrical plug-in receptacle.
Typically, the pumping unit is disposed within the well bore just above where perforations are made into a hydrocarbon producing zone. This placement thereby allows the produced fluids to flow past the outer surface of the pumping motor and provide a cooling effect.
In spite of the heat transfer between the fluid and the motor, over a period of time the motor may become overheated. Overheating may a problem when the fluid has a high viscosity, a low specific heat, and a low thermal conductivity. This is typical of highly viscous crude oils. Also, the motor may be forced to operate at an elevated temperature, past its normal operating temperature, in steam injection wells. Elevated well temperatures can reduce motor life.